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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group Luca Martini 3 Internet Draft Nasser El-Aawar 4 Expiration Date: May 2002 Level 3 Communications, LLC. 6 Steve Vogelsang Daniel Tappan 7 John Shirron Eric C. Rosen 8 Toby Smith Alex Hamilton 9 Laurel Networks, Inc. Jayakumar Jayakumar 10 Cisco Systems, Inc. 12 Vasile Radoaca Dimitri Stratton Vlachos 13 Nortel Networks Mazu Networks, Inc. 15 Andrew G. Malis Chris Liljenstolpe 16 Vinai Sirkay Cable & Wireless 17 Vivace Networks, Inc. 18 Giles Heron 19 Dave Cooper PacketExchange Ltd. 20 Global Crossing 21 Kireeti Kompella 22 Juniper Networks 24 November 2001 26 Encapsulation Methods for Transport of Layer 2 Frames Over IP and MPLS Networks 28 draft-martini-l2circuit-encap-mpls-04.txt 30 Status of this Memo 32 This document is an Internet-Draft and is in full conformance with 33 all provisions of Section 10 of RFC2026. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF), its areas, and its working groups. Note that other 37 groups may also distribute working documents as Internet-Drafts. 39 Internet-Drafts are draft documents valid for a maximum of six months 40 and may be updated, replaced, or obsoleted by other documents at any 41 time. It is inappropriate to use Internet-Drafts as reference 42 material or to cite them other than as "work in progress." 44 The list of current Internet-Drafts can be accessed at 45 http://www.ietf.org/ietf/1id-abstracts.txt. 47 The list of Internet-Draft Shadow Directories can be accessed at 48 http://www.ietf.org/shadow.html. 50 Abstract 52 This document describes methods for encapsulating the Protocol Data 53 Units (PDUs) of layer 2 protocols such as Frame Relay, ATM, or 54 Ethernet for transport across an MPLS or IP network. 56 Table of Contents 58 1 Specification of Requirements .......................... 3 59 2 Introduction ........................................... 3 60 3 General encapsulation method ........................... 4 61 3.1 The Control Word ....................................... 4 62 3.1.1 Setting the sequence number ............................ 5 63 3.1.2 Processing the sequence number ......................... 5 64 3.2 MTU Requirements ....................................... 6 65 4 Protocol-Specific Details .............................. 7 66 4.1 Frame Relay ............................................ 7 67 4.2 ATM .................................................... 8 68 4.2.1 ATM AAL5 CPCS-SDU Mode ................................. 8 69 4.2.2 ATM Cell Mode .......................................... 10 70 4.2.3 OAM Cell Support ....................................... 11 71 4.2.4 CLP bit to Quality of Service mapping .................. 12 72 4.3 Ethernet VLAN .......................................... 12 73 4.4 Ethernet ............................................... 12 74 4.5 HDLC ................................................... 13 75 4.6 PPP .................................................... 13 76 5 Using an MPLS Label as the Demultiplexer Field ......... 13 77 5.1 MPLS Shim EXP Bit Values ............................... 14 78 5.2 MPLS Shim S Bit Value .................................. 14 79 5.3 MPLS Shim TTL Values ................................... 14 80 6 Security Considerations ................................ 14 81 7 Intellectual Property Disclaimer ....................... 14 82 8 References ............................................. 14 83 9 Author Information ..................................... 15 85 1. Specification of Requirements 87 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 88 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 89 document are to be interpreted as described in RFC 2119 91 2. Introduction 93 In an MPLS or IP network, it is possible to use control protocols 94 such as those specified in [1] to set up "emulated virtual circuits" 95 that carry the the Protocol Data Units of layer 2 protocols across 96 the network. A number of these emulated virtual circuits may be 97 carried in a single tunnel. This requires of course that the layer 2 98 PDUs be encapsulated. We can distinguish three layers of this 99 encapsulation: 101 - the "tunnel header", which contains the information needed to 102 transport the PDU across the IP or MPLS network; this is header 103 belongs to the tunneling protocol, e.g., MPLS, GRE, L2TP. 105 - the "demultiplexer field", which is used to distinguish 106 individual emulated virtual circuits within a single tunnel; this 107 field must be understood by the tunneling protocol as well; it 108 may be, e.g., an MPLS label or a GRE key field. 110 - the "emulated VC encapsulation", which contains the information 111 about the enclosed layer 2 PDU which is necessary in order to 112 properly emulate the corresponding layer 2 protocol. 114 This document specifies the emulated VC encapsulation for a number of 115 layer 2 protocols. Although different layer 2 protocols require 116 different information to be carried in this encapsulation, an attempt 117 has been made to make the encapsulation as common as possible for all 118 layer 2 protocols. 120 This document also specifies the way in which the demultiplexer field 121 is added to the emulated VC encapsulation when an MPLS label is used 122 as the demultiplexer field. 124 QoS related issues are not discussed in this draft 126 For the purpose of this document R1 will be defined as the ingress 127 router, and R2 as the egress router. A layer 2 PDU will be received 128 at R1, encapsulated at R1, transported, decapsulated at R2, and 129 transmitted out of R2. 131 3. General encapsulation method 133 In most cases, it is not necessary to transport the layer 2 134 encapsulation across the network; rather, the layer 2 header can be 135 stripped at R1, and reproduced at R2. This is done using information 136 carried in the control word (see below), as well as information that 137 may already have been signaled from R1 to R2. 139 3.1. The Control Word 141 There are three requirements that may need to be satisfied when 142 transporting layer 2 protocols over an IP or MPLS backbone: 144 -i. Sequentiality may need to be preserved. 145 -ii. Small packets may need to be padded in order to be 146 transmitted on a medium where the minimum transport unit is 147 larger than the actual packet size. 148 -iii. Control bits carried in the header of the layer 2 frame may 149 need to be transported. 151 The control word defined here addresses all three of these 152 requirements. For some protocols this word is REQUIRED, and for 153 others OPTIONAL. For protocols where the control word is OPTIONAL 154 implementations MUST support sending no control word, and MAY support 155 sending a control word. 157 In all cases the egress router must be aware of whether the ingress 158 router will send a control word over a specific virtual circuit. 159 This may be achieved by configuration of the routers, or by 160 signaling, for example as defined in [1]. 162 The control word is defined as follows: 164 0 1 2 3 165 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 166 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 167 | Rsvd | Flags |0 0| Length | Sequence Number | 168 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 170 In the above diagram the first 4 bits are reserved for future use. 171 They MUST be set to 0 when transmitting, and MUST be ignored upon 172 receipt. 174 The next 4 bits provide space for carrying protocol specific flags. 175 These are defined in the protocol-specific details below. 177 The next 2 bits MUST be set to 0 when transmitting. 179 The next 6 bits provide a length field, which is used as follows: If 180 the packet's length (defined as the length of the layer 2 payload 181 plus the length of the control word) is less than 64 bytes, the 182 length field MUST be set to the packet's length. Otherwise the length 183 field MUST be set to zero. The value of the length field, if non- 184 zero, can be used to remove any padding. When the packet reaches the 185 service provider's egress router, it may be desirable to remove the 186 padding before forwarding the packet. 188 The next 16 bits provide a sequence number that can be used to 189 guarantee ordered packet delivery. The processing of the sequence 190 number field is OPTIONAL. 192 The sequence number space is a 16 bit, unsigned circular space. The 193 sequence number value 0 is used to indicate an unsequenced packet. 195 3.1.1. Setting the sequence number 197 For a given emulated VC, and a pair of routers R1 and R2, if R1 198 supports packet sequencing then the following procedures should be 199 used: 201 - the initial packet transmitted on the emulated VC MUST use 202 sequence number 1 203 - subsequent packets MUST increment the sequence number by one for 204 each packet 205 - when the transmit sequence number reaches the maximum 16 bit 206 value (65535) the sequence number MUST wrap to 1 208 If the transmitting router R1 does not support sequence number 209 processing, then the sequence number field in the control word MUST 210 be set to 0. 212 3.1.2. Processing the sequence number 214 If a router R2 supports receive sequence number processing, then the 215 following procedures should be used: 217 When an emulated VC is initially set up, the "expected sequence 218 number" associated with it MUST be initialized to 1. 220 When a packet is received on that emulated VC, the sequence number 221 should be processed as follows: 223 - if the sequence number on the packet is 0, then the packet passes 224 the sequence number check 226 - otherwise if the packet sequence number >= the expected sequence 227 number and the packet sequence number - the expected sequence 228 number < 32768, then the packet is in order. 230 - otherwise if the packet sequence number < the expected sequence 231 number and the expected sequence number - the packet sequence 232 number >= 32768, then the packet is in order. 234 - otherwise the packet is out of order. 236 If a packet passes the sequence number check, or is in order then, it 237 can be delivered immediately. If the packet is in order, then the 238 expected sequence number should be set using the algorithm: 240 expected_sequence_number := packet_sequence_number + 1 mod 2**16 241 if (expected_sequence_number = 0) then expected_sequence_number := 1; 243 Packets which are received out of order MAY be dropped or reordered 244 at the discretion of the receiver. 246 If a router R2 does not support receive sequence number processing, 247 then the sequence number field MAY be ignored. 249 3.2. MTU Requirements 251 The network MUST be configured with an MTU that is sufficient to 252 transport the largest encapsulation frames. If MPLS is used as the 253 tunneling protocol, for example, this is likely to be 12 or more 254 bytes greater than the largest frame size. Other tunneling protocols 255 may have longer headers and require larger MTUs. If the ingress 256 router determines that an encapsulated layer 2 PDU exceeds the MTU of 257 the tunnel through which it must be sent, the PDU MUST be dropped. If 258 an egress router receives an encapsulated layer 2 PDU whose payload 259 length (i.e., the length of the PDU itself without any of the 260 encapsulation headers), exceeds the MTU of the destination layer 2 261 interface, the PDU MUST be dropped. 263 4. Protocol-Specific Details 265 4.1. Frame Relay 267 A Frame Relay PDU is transported without the Frame Relay header or 268 the FCS. The control word is REQUIRED; however, its use is optional, 269 although desirable. Use of the control word means that the ingress 270 and egress LSRs follow the procedures below. If an ingress LSR 271 chooses not to use the control word, it MUST set the flags in the 272 control word to 0; if an egress LSR chooses to ignore the control 273 word, it MUST set the Frame Relay control bits to 0. 275 The BECN, FECN, DE and C/R bits are carried across the network in the 276 control word. The edge routers that implement this document MAY, when 277 either adding or removing the encapsulation described herein, change 278 the BECN and/or FECN bits from zero to one in order to reflect 279 congestion in the network that is known to the edge routers, and the 280 D/E bit from zero to one to reflect marking from edge policing of the 281 Frame Relay Committed Information Rate. The BECN, FECN, and D/E bits 282 SHOULD NOT be changed from one to zero. 284 The following is an example of a Frame Relay packet: 286 0 1 2 3 287 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 288 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 289 | Rsvd |B|F|D|C| Length | Sequence Number | 290 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 291 | Frame Relay PDU | 292 | " | 293 | " | 294 | " | 295 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 297 * B ( BECN ) Bit 299 The ingress router, R1, SHOULD copy the BECN field from the 300 incoming Frame Relay header into this field. The egress router, 301 R2, MUST generate a new BECN field based on the value of the B 302 bit. 304 * F ( FECN ) Bit 306 The ingress router, R1, SHOULD copy the FECN field from the 307 incoming Frame Relay header into this field. The egress router, 308 R2, MUST generate a new FECN field based on the value of the F 309 bit. 311 * D ( DE ) Bit 313 The ingress router, R1, SHOULD copy the DE field from the 314 incoming Frame Relay header into this field. The egress router, 315 R2, MUST generate a new DE field based on the value of the D bit. 317 If the tunneling protocol provides a field which can be set to 318 specify a Quality of Service, the ingress router, R1, MAY 319 consider the DE bit of the Frame Relay header when determining 320 the value of that field. The egress router MAY then consider the 321 value of this field when queuing the layer 2 PDU for egress. 322 Note however that frames from the same VC MUST NOT be reordered. 324 * C ( C/R ) Bit 326 The ingress router, R1, SHOULD copy the C/R bit from the received 327 Frame Relay PDU to the C bit of the control word. The egress 328 router, R2, MUST copy the C bit into the output frame. 330 4.2. ATM 332 Two encapsulations are supported for ATM transport: one for ATM AAL5 333 and another for ATM cells. 335 The AAL5 CPCS-SDU encapsulation consists of the REQUIRED control 336 word, and the AAL5 CPCS-SDU. The ATM cell encapsulation consists of 337 an OPTIONAL control word, a 4 byte ATM cell header, and the ATM cell 338 payload. 340 4.2.1. ATM AAL5 CPCS-SDU Mode 342 In ATM AAL5 mode the ingress router is required to reassemble AAL5 343 CPCS-SDUs from the incoming VC and transport each CPCS-SDU as a 344 single packet. No AAL5 trailer is transported. The control word is 345 REQUIRED; its use, however, is optional, although desirable. Use of 346 the control word means that the ingress and egress LSRs follow the 347 procedures below. If an ingress LSR chooses not to use the control 348 word, it MUST set the flags in the control word to 0; if an egress 349 LSR chooses to ignore the control word, it MUST set the ATM control 350 bits to 0. 352 The EFCI and CLP bits are carried across the network in the control 353 word. The edge routers that implement this document MAY, when either 354 adding or removing the encapsulation described herein, change the 355 EFCI bit from zero to one in order to reflect congestion in the 356 network that is known to the edge routers, and the CLP bit from zero 357 to one to reflect marking from edge policing of the ATM Sustained 358 Cell Rate. The EFCI and CLP bits MUST NOT be changed from one to 359 zero. 361 The AAL5 CPCS-SDU is prepended by the following header: 363 0 1 2 3 364 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 365 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 366 | Rsvd |T|E|L|C| Length | Sequence Number | 367 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 368 | ATM AAL5 CPCS-SDU | 369 | " | 370 | " | 371 | " | 372 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 374 * T (transport type) bit 376 Bit (T) of the control word indicates whether the packet contains 377 an ATM cell or an AAL5 CPCS-SDU. If set the packet contains an 378 ATM cell, encapsulated according to the ATM cell mode section 379 below, otherwise it contains an AAL5 CPCS-SDU. The ability to 380 transport an ATM cell in the AAL5 mode is intended to provide a 381 means of enabling OAM functionality over the AAL5 VC. 383 * E ( EFCI ) Bit 385 The ingress router, R1, SHOULD set this bit to 1 if the EFCI bit 386 of the final cell of those that transported the AAL5 CPCS-SDU is 387 set to 1, or if the EFCI bit of the single ATM cell to be 388 transported in the packet is set to 1. Otherwise this bit 389 SHOULD be set to 0. The egress router, R2, SHOULD set the EFCI 390 bit of all cells that transport the AAL5 CPCS-SDU to the value 391 contained in this field. 393 * L ( CLP ) Bit 395 The ingress router, R1, SHOULD set this bit to 1 if the CLP bit 396 of any of the ATM cells that transported the AAL5 CPCS-SDU is set 397 to 1, or if the CLP bit of the single ATM cell to be transported 398 in the packet is set to 1. Otherwise this bit SHOULD be set to 399 0. The egress router, R2, SHOULD set the CLP bit of all cells 400 that transport the AAL5 CPCS-SDU to the value contained in this 401 field. 403 * C ( Command / Response Field ) Bit 405 When FRF.8.1 Frame Relay / ATM PVC Service Interworking [3] 406 traffic is being transported, the CPCS-UU Least Significant Bit 407 (LSB) of the AAL5 CPCS-SDU may contain the Frame Relay C/R bit. 408 The ingress router, R1, SHOULD copy this bit to the C bit of the 409 control word. The egress router, R2, SHOULD copy the C bit to the 410 CPCS-UU Least Significant Bit (LSB) of the AAL5 CPCS PDU. 412 4.2.2. ATM Cell Mode 414 In this encapsulation mode ATM cells are transported individually 415 without a SAR process. The ATM cell encapsulation consists of an 416 OPTIONAL control word, and one or more ATM cells - each consisting of 417 a 4 byte ATM cell header and the 48 byte ATM cell payload. This ATM 418 cell header is defined as in the FAST encapsulation [4] section 419 3.1.1, but without the trailer byte. The length of each frame, 420 without the encapsulation headers, is a multiple of 52 bytes long. 421 The maximum number of ATM cells that can be fitted in a frame, in 422 this fashion, is limited only by the network MTU and by the ability 423 of the egress router to process them. The ingress router MUST NOT 424 send more cells than the egress router is willing to receive. The 425 number of cells that the egress router is willing to receive may 426 either be configured in the ingress router or may be signaled, for 427 example using the methods described in [1]. The number of cells 428 encapsulated in a particular frame can be inferred by the frame 429 length. The control word is OPTIONAL. If the control word is used 430 then the flag bits in the control word are not used, and MUST be set 431 to 0 when transmitting, and MUST be ignored upon receipt. 433 The EFCI and CLP bits are carried across the network in the ATM cell 434 header. The edge routers that implement this document MAY, when 435 either adding or removing the encapsulation described herein, change 436 the EFCI bit from zero to one in order to reflect congestion in the 437 network that is known to the edge router, and the CLP bit from zero 438 to one to reflect marking from edge policing of the ATM Sustained 439 Cell Rate. The EFCI and CLP bits SHOULD NOT be changed from one to 440 zero. 442 This diagram illustrates an encapsulation of two ATM cells: 444 0 1 2 3 445 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 446 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 447 | Control word ( Optional ) | 448 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 449 | VPI | VCI | PTI |C| 450 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 451 | ATM Payload ( 48 bytes ) | 452 | " | 453 | " | 454 | " | 455 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 456 | VPI | VCI | PTI |C| 457 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 458 | ATM Payload ( 48 bytes ) | 459 | " | 460 | " | 461 | " | 462 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 464 * VPI 466 The ingress router MUST copy the VPI field from the incoming cell 467 into this field. For particular emulated VCs, the egress router 468 MAY generate a new VPI and ignore the VPI contained in this 469 field. 471 * VCI 473 The ingress router MUST copy the VCI field from the incoming ATM 474 cell header into this field. For particular emulated VCs, the 475 egress router MAY generate a new VCI. 477 * PTI & CLP ( C bit ) 479 The PTI and CLP fields are the PTI and CLP fields of the incoming 480 ATM cells. The cell headers of the cells within the packet are 481 the ATM headers (without HEC) of the incoming cell. 483 4.2.3. OAM Cell Support 485 OAM cells MAY be transported on the VC LSP. An egress router that 486 does not support transport of OAM cells MUST discard frames that 487 contain an ATM cell with the high-order bit of the PTI field set to 488 1. A router that supports transport of OAM cells MUST follow the 489 procedures outlined in [4] section 8 for mode 0 only, in addition to 490 the applicable procedures specified in [1]. 492 4.2.4. CLP bit to Quality of Service mapping 494 The ingress router MAY consider the CLP bit when determining the 495 value to be placed in the Quality of Service fields (e.g. the EXP 496 fields of the MPLS label stack) of the encapsulating protocol. This 497 gives the network visibility of the CLP bit. Note however that cells 498 from the same VC MUST NOT be reordered. 500 4.3. Ethernet VLAN 502 For an Ethernet 802.1q VLAN the entire Ethernet frame without the 503 preamble or FCS is transported as a single packet. The control word 504 is OPTIONAL. If the control word is used then the flag bits in the 505 control word are not used, and MUST be set to 0 when transmitting, 506 and MUST be ignored upon receipt. The 4 byte VLAN tag is transported 507 as is, and MAY be overwritten by the egress router. 509 The ingress router MAY consider the user priority field [5] of the 510 VLAN tag header when determining the value to be placed in the 511 Quality of Service field of the encapsulating protocol (e.g., the EXP 512 fields of the MPLS label stack). In a similar way, the egress router 513 MAY consider the Quality of Service field of the encapsulating 514 protocol when queuing the packet for egress. Ethernet packets 515 containing hardware level CRC errors, framing errors, or runt packets 516 MUST be discarded on input. 518 4.4. Ethernet 520 For simple Ethernet port to port transport, the entire Ethernet frame 521 without the preamble or FCS is transported as a single packet. The 522 control word is OPTIONAL. If the control word is used then the flag 523 bits in the control word are not used, and MUST be set to 0 when 524 transmitting, and MUST be ignored upon receipt. As in the Ethernet 525 VLAN case, Ethernet packets with hardware level CRC errors, framing 526 errors, and runt packets MUST be discarded on input. 528 4.5. HDLC 530 HDLC mode provides port to port transport of HDLC encapsulated 531 traffic. The HDLC PDU is transported in its entirety, including the 532 HDLC address, control and protocol fields, but excluding HDLC flags 533 and the FCS. Bit/Byte stuffing is undone. The control word is 534 OPTIONAL. If the control word is used then the flag bits in the 535 control word are not used, and MUST be set to 0 when transmitting, 536 and MUST be ignored upon receipt. 538 The HDLC mode is suitable for port to port transport of Frame Relay 539 UNI or NNI traffic. It must be noted, however, that this mode is 540 transparent to the FECN, BECN and DE bits. 542 4.6. PPP 544 PPP mode provides point to point transport of PPP encapsulated 545 traffic, as specified in [6]. The PPP PDU is transported in its 546 entirety, including the protocol field (whether compressed using PFC 547 or not), but excluding any media-specific framing information, such 548 as HDLC address and control fields or FCS. Since media-specific 549 framing is not carried the following options will not operate 550 correctly if the PPP peers attempt to negotiate them: 552 - Frame Check Sequence (FCS) Alternatives 553 - Address-and-Control-Field-Compression (ACFC) 554 - Asynchronous-Control-Character-Map (ACCM) 556 Note also that VC LSP Interface MTU negotiation as specified in [1] 557 is not affected by PPP MRU advertisement. Thus if a PPP peer sends a 558 PDU with a length in excess of that negotiated for the VC LSP that 559 PDU will be discarded by the ingress router. 561 The control word is OPTIONAL. If the control word is used then the 562 flag bits in the control word are not used, and MUST be set to 0 when 563 transmitting, and MUST be ignored upon receipt. 565 5. Using an MPLS Label as the Demultiplexer Field 567 To use an MPLS label as the demultiplexer field, a 32-bit label stack 568 entry [2] is simply prepended to the emulated VC encapsulation, and 569 hence will appear as the bottom label of an MPLS label stack. This 570 label may be called the "VC label". The particular emulated VC 571 identified by a particular label value must be agreed by the ingress 572 and egress LSRs, either by signaling (e.g, via the methods of [1]) or 573 by configuration. Other fields of the label stack entry are set as 574 follows. 576 5.1. MPLS Shim EXP Bit Values 578 If it is desired to carry Quality of Service information, the Quality 579 of Service information SHOULD be represented in the EXP field of the 580 VC label. If more than one MPLS label is imposed by the ingress LSR, 581 the EXP field of any labels higher in the stack SHOULD also carry the 582 same value. 584 5.2. MPLS Shim S Bit Value 586 The ingress LSR, R1, MUST set the S bit of the VC label to a value of 587 1 to denote that the VC label is at the bottom of the stack. 589 5.3. MPLS Shim TTL Values 591 The ingress LSR, R1, SHOULD set the TTL field of the VC label to a 592 value of 2. 594 6. Security Considerations 596 This document specifies only encapsulations, and not the protocols 597 used to carry the encapsulated packets across the network. Each such 598 protocol may have its own set of security issues, but those issues 599 are not affected by the encapsulations specified herein. 601 7. Intellectual Property Disclaimer 603 This document is being submitted for use in IETF standards 604 discussions. 606 8. References 608 [1] "Transport of Layer 2 Frames Over MPLS", draft-martini- 609 l2circuit-trans-mpls-08.txt. ( work in progress ) 611 [2] "MPLS Label Stack Encoding", E. Rosen, Y. Rekhter, D. Tappan, G. 612 Fedorkow, D. Farinacci, T. Li, A. Conta. RFC3032 614 [3] "Frame Relay / ATM PVC Service Interworking Implementation 615 Agreement", Frame Relay Forum 2000. 617 [4] "Frame Based ATM over SONET/SDH Transport (FAST)," 2000. 619 [5] "IEEE 802.3ac-1998" IEEE standard specification. 621 [6] "The Point-to-Point Protocol (PPP)", RFC 1661. 623 9. Author Information 625 Luca Martini 626 Level 3 Communications, LLC. 627 1025 Eldorado Blvd. 628 Broomfield, CO, 80021 629 e-mail: luca@level3.net 631 Nasser El-Aawar 632 Level 3 Communications, LLC. 633 1025 Eldorado Blvd. 634 Broomfield, CO, 80021 635 e-mail: nna@level3.net 637 Giles Heron 638 PacketExchange Ltd. 639 The Truman Brewery 640 91 Brick Lane 641 LONDON E1 6QL 642 United Kingdom 643 e-mail: giles@packetexchange.net 645 Dimitri Stratton Vlachos 646 Mazu Networks, Inc. 647 125 Cambridgepark Drive 648 Cambridge, MA 02140 649 e-mail: d@mazunetworks.com 651 Dan Tappan 652 Cisco Systems, Inc. 653 250 Apollo Drive 654 Chelmsford, MA, 01824 655 e-mail: tappan@cisco.com 656 Jayakumar Jayakumar, 657 Cisco Systems Inc. 658 225, E.Tasman, MS-SJ3/3, 659 San Jose , CA, 95134 660 e-mail: jjayakum@cisco.com 662 Alex Hamilton, 663 Cisco Systems Inc. 664 285 W. Tasman , MS-SJCI/3/4, 665 San Jose, CA, 95134 666 e-mail: tahamilt@cisco.com 668 Eric Rosen 669 Cisco Systems, Inc. 670 250 Apollo Drive 671 Chelmsford, MA, 01824 672 e-mail: erosen@cisco.com 674 Steve Vogelsang 675 Laurel Networks, Inc. 676 Omega Corporate Center 677 1300 Omega Drive 678 Pittsburgh, PA 15205 679 e-mail: sjv@laurelnetworks.com 681 John Shirron 682 Laurel Networks, Inc. 683 Omega Corporate Center 684 1300 Omega Drive 685 Pittsburgh, PA 15205 686 e-mail: jshirron@laurelnetworks.com 688 Toby Smith 689 Laurel Networks, Inc. 690 Omega Corporate Center 691 1300 Omega Drive 692 Pittsburgh, PA 15205 693 e-mail: tob@laurelnetworks.com 694 Andrew G. Malis 695 Vivace Networks, Inc. 696 2730 Orchard Parkway 697 San Jose, CA 95134 698 e-mail: Andy.Malis@vivacenetworks.com 700 Vinai Sirkay 701 Vivace Networks, Inc. 702 2730 Orchard Parkway 703 San Jose, CA 95134 704 e-mail: sirkay@technologist.com 706 Vasile Radoaca 707 Nortel Networks 708 600 Technology Park 709 Billerica MA 01821 710 e-mail: vasile@nortelnetworks.com 712 Chris Liljenstolpe 713 Cable & Wireless 714 11700 Plaza America Drive 715 Reston, VA 20190 716 e-mail: chris@cw.net 718 Dave Cooper 719 Global Crossing 720 960 Hamlin Court 721 Sunnyvale, CA 94089 722 e-mail: dcooper@gblx.net 724 Kireeti Kompella 725 Juniper Networks 726 1194 N. Mathilda Ave 727 Sunnyvale, CA 94089 728 e-mail: kireeti@juniper.net